The main way the big names in free software make money is through paid support.
As mentioned by ryan, although all of the code is open, Red Hat charge for pre-built binaries (for RHEL), they also charge for support too. So companies and individuals pay an annual subscription fee which entitles them to live support (phone, IRC/chat, Skype etc). So they can get instant answers/solutions to any problems that they are experiencing.
So offering users paid support is one way that you can make money from free software. You have mentioned that you are autistic, so this might not be for you, but it is the main way that I am aware of that money can be made from free software!
Another way, albeit not guaranteed to bring in any significant sum, would be to set up a way for people to donate money to your project via paypal or some other online payment system.
Because this relies on the generosity of others, you are not guaranteed any kind of regular income from this. But you might still get a few donations. And if your software is good enough, it could give people an incentive to donate larger sums. I have worked for companies in the past who have donated money to the developers of useful pieces of free software. And the donations ranged anywhere from a few pounds, right up to a few thousand pounds!
But without a donation page, you have no chance of getting anything. So setting up a donation page for your free-software project is a no-brainer!
Another way to make money is through selling manuals and training. Good quality free-software deserves equally good-quality free documentation. So you should offer a decent, free manual for your software out of courtesy. But there is nothing to stop you selling printed hard-copies of your manual, or selling other training materials which will teach people to use your software effectively.
A good example of this would be The Blender Foundation. Blender is an incredible (and incredibly complex) piece of free/open source software. There is a tremendous amount of free documentation available for Blender. But the Blender Foundation also sell DVD's, books and even live, classroom based training courses to teach people how to use Blender. So documentation and training material is another potential revenue stream for a free software project.
If you have a big enough user-base, another way to make a little money (albeit not guaranteed) is to sell merchandise branded with your projects logo - T-shirts, mugs, keyrings, pocket protectors, USB drives etc etc.
So, off the top of my head that's five potential revenue streams for free-software companies:
1. Sale of pre-built binary packages
2. Offering paid support
3. Donations
4. Sale of documentation/training materials
5. Sale of branded merchandise
Depending on the scope / complexity of your project, some of these revenue streams might not be open to you. If you have a simple command-line program, with limited scope and usage: Your users won't need much in the way of documentation or training; So your free documentation will suffice. Users will also not need to pay for any kind of support. Selling pre-built binaries is probably also pointless in this case too, which would leave you relying on donations and perhaps selling merchandise!
Whereas if your project is more complex and powerful (like an entire Linux distro, or an audio/video editor, or a CAD package etc.), then you have more scope for opening other revenue streams from it!